Among steel pipes for electric resistance welded oil well pipes, API 5CT K55 (hereinafter may be simply referred to as “K55”) is a steel pipe that satisfies TS (tensile strength)>655 N/mm2 and YS (yield strength)=from 379 to 552 N/mm2, and API 5CT J55 (hereinafter may be simply referred to as “J55”) is a steel pipe that satisfies TS>517 N/mm2 and YS=level equivalent to YS of K55. K55 has a feature in that the yield ratio (YR) of K55 is lower than that of J55 (for example, YR of K55 is less than 80%) because the lower limit of TS of K55 is higher than that of J55.
The following Patent Documents 1 to 6 describe K55 or steel sheets used in K55.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. H07-102321    Patent Document 2: International Publication No. WO 2012/144248    Patent Document 3: JP-A No. S61-048518    Patent Document 4: JP-A No. 2011-089152    Patent Document 5: JP-A No. 2012-132060    Patent Document 6: International Publication No. WO 2013/153676